Fluid pressure system and control therefor



Apri l5, 1941. c. M. KENDRICK FLUID `PRESSURE SYSTEM AND CONTROLTHEREFOR Filed May 12, 1938 4 Sheets-Sheet l N www. 3mm mmmwm Mmmm .h wmR.

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April 15, 1941.

c. M. KENDmcK `FLUID PRESSURE SYS'IEM- AND CONTROL THEREFOR Filed layA12, 1958 4 Sheets-Sheet 2 A TTORNEYS.

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pril 15, 1941,

FLUID PRESSURE SYSTEM AND.CONTROL THREFOR @ELL Q I' Y Il' sv QN .w f NQN o EN @QN \N w N .www

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W w M Patented Apr. 15, 1941 UNITED STATES PATENT OFFICE FLU'I/`PRESSURE SYSTEM AND CONTROL THEREFOR I Cha yles M. Kendrick, New York,Y., assignor to Manly Corporation, Washington, D. C., a corporation ofDelaware Application May 12, 1938, Serial No. 207,512

18 Claims.

This invention relates to hydrostatic fluid pressure systems whereiniluid pressure operated devices are supplied with fluid under pressureby variable delivery pumps and relates more particularly to means forcontrolling the outputs of pumps, although certain features thereof haveother applications.

One object of the invention is a novel andimproved system of thischaracter and controls.

variable delivery pump, and leakage therein, etc.

A further object of the invention is a system oi this character wherebyinnite control oi the output of the variable delivery pump and the.operation of the fluid pressure operated devices may be effected andthe output of the pump may be maintained substantially constant for anyparticular adjustment.

ii further object is a control of this character whereby the output ofthe pump will beheld substantially constant at the output orrate offluid flow determined by the control system irrespectivevof changes inoperating pressure, temperature, speed of operation and other operatingconditions which are subject to change during the course of operationand which aiiect the variable delivery pumps output.

A still further object is to provide an output control for variabledelivery pumps in which a metering orifice is employed together withmeans for compensating for the effect of change in viscosity of thecirculated fluid upon the difference in presures on the inlet and outletsides of said orice, whereby such viscosity change doesv tion, of anillustrative embodiment of the present invention;

Fig. 1a is a diagrammatic view, pagtly in secironof a modified form ofthe embodiment of Fig. 2 is a diagrammatic view, partly in section,showing another modification; and

Figs. 3 to 6 are diagrammatic views, also partly in section, of othermodifications of the invention embodying different embodiments oiviscosity compensating means.

The invention is illustrated as applied to an hydraulic transmissionsystem including a huid actuated motor or driven device it and in whicha liquid. such as oil, is employed ior operating the driven device. 'iheiiuid pressure system is a hydrostatic nuid pressure system in which allcontrol or regulating mechanism is responsive to `the hydrostaticpressure or pressures in said system, as distinguished from systems thatinclude jets or other mechanisms employing the lrinetic energy oi themoving fluid in their operation. The fluid circuit includes'a variabledelivery pp I0` which supplies the motive Huid ior operating the drivendevice iii, and the operation oi the latter may be controlled bymanipulating a How resistance means included in the iiuid circuit, hereshown as a variable orince it. The drawings `show several modificationsoi the invention and in these modifications the delivery oi the pump i0and hence the operation of the motor it may be varied to any desiredextent from minimum to maximum, this variation being efiected either byvarying the extent of opening oi the orifice it' or by varying theamount oi pressure drop to be maintained across said orifice, or by bothmethods. These modifications embody control mechanisms which respond tothese manipulations to adjust the delivery oi the pump I0 to the exactrequirements determined thereby and to maintain the output oi the pumpl0 at the value predetermined by any particular setting or adjustment.

Referring now these drawings, the hydrostatic fluid pressure systemillustrated in Fig. 1 includes a variable delivery pump I0 receiving itssupply of fluid through an inlet conduit II from a reservoir I2. Thepump I0 is also connected with a discharge conduit having a portion I3through which fluid under operating pressure is conveyed to the fluidactuated motor or driven member I4 and also having a portion I5 throughwhich the uid exhausted by the driven member I4 returns to the reservoirI2.

The volume of fluid delivered per revolution or other unit of movementof the driving element of the pump I is controlled in the embodimentshown by the position of a laterally movable adjusting rod orvolume-determining element l1. The arrangement is such that movement ofthe adjusting rod li inward, or toward the right as viewed in Fig. l,causes decrease in the delivered volume, while its movement outward, ortoward the left, causes increase in said delivered volume.

The adjusting element l1 is moved and its position controlled by poweractuated means in co-operation with control mechanism. The poweractuated means in this instance is a fluid motor which includes anadjusting piston 2li reciprocable in an adjusting cylinder 2l andappropriately connected with the adjusting rod li, as indicated at i3,for simultaneous movement therewith.

Movement of the adjusting piston 20, and hence its position, iscontrolled by admission of pressure iiuid to one end of the adjustingcylinder 2i and simultaneous exhaust of fluid from the other end thereofunder control of valve mechanism. For this purpose the ends of theadjusting cylinder 2l are suitably connected, as by passages 23 and 24respectively, with a pair of annular cylinder ports 2 and 28 in the bore26 of the valve housing 25, the valve bore 26 being appropriately closedat both of its ends. Admission and exhaust of iiuid to and from the endsof the adjusting cylinder 2l are regulated by a valve piston 3Q slidablewithin the Valve bore 26 and provided with a pair of heads 3| and 32which are separated by a reduced portion 33. The arrangement is suchthat the heads 3l and 32 cover the cylinder ports 21 and 28 respectivelywhen the valve piston 30 is in its neutral position in which it is shownin Fig. 1 and in which said heads 3| and 32 cut ofi all fluidcommunication between the ends of the adjusting cylinder 2l and thevalve bore 26, so that the adjusting piston is rendered inoperative.Movement of the valve piston 3G away from its neutral position andtoward the right as viewed in Fig. l will cause the adjusting piston 20to move outward or toward the left; and, similarly, movement of theValve piston .ill away from its neutral position and toward the leftwill cause the adjusting piston 2li to move inward or toward the right,as will be later explained in more detail.

Movement and position of the valve piston Sil are controlled responsiveto the actual output of the pump lil with respect to a predeterminedoutput thereof. In the preferred embodiment of this invention aconventional metering orifice d is positioned in the portion i3 of thedischarge conduit, that is, in the portion of the discharge conduitintermediate the pump l0 and the driven member i4. The orifice 40 is ofthe variable type and may be infinitely adjusted to any size or extentof opening from its minimum (such as its zero or fully closed position)to its maximum or fully open position by any suitable means, as forinstance schematically illustrated at 4l, and actuated in anyappropriate manner such, for example, as manually.

The opposite ends of the valve piston 30, which are of equal area, areadapted to be acted upon by uid from the inlet and outlet sidesrespectively of the orice 40 in order that said valve pistons movementmay be responsive to variation in and its position corresponding to theactual rate of fluid flow through said orifice l with respect to thepredetermined rate; that is to say, position and movement of the valvepiston are determined by the actual pressure drop across the orifice l0with respect to a predetermined pressure drop thereacross. For thispurpose the opposite ends of the valve bore 26 are supplied with :duidfrom the inlet and outlet sides respectively of the orifice lill, as bythe passages 42 and d3 which enter said valve bore 26 at pointsintermediate its closed ends and the ends of the valve piston Sil. Withthis arrangement, pressure fluid from the inlet side of the orice 60entering the right hand end of the valve bore 26 exerts a force upon thevalve piston 3i) tending to move it towardv the left as viewed in Fig.l, this force being opposed by the action of pressure fiuid from theoutlet side of the orifice d upon the left hand end of the valve piston3i] which, of course, tends to move said valve piston toward the right.

The force exerted upon the valve piston 30 by pressure iuid from theoutlet side of the orifice Sii, that is, the force tending to move thevalve piston 3l) toward the right, is supplemented by :force exerted bya compression spring 34 positioned in the left hand end of the valvebore 26. One end of the spring 3l! bears against the adjacent end of thevalve piston 30 and its other end bears against an abutment piece 35disposed intermediate the end of said spring 34 and the end of a screw36 which extends through the closed end of the valve bore 26 andprovides means by which the compression of the spring 3d may beadjusted. The force exerted by the spring 3i thus combines with theduid-exerted force tending to move the valve piston 30 toward the rightand together they act to oppose and balance the single force exertedupon said valve piston 30 by iiuid from the inlet side of the orice d0,as will be more fully explained presently.

Movement of the Valve piston 30 in the valve bore 26 is preferablylimited in order to assure that both ends of the valve piston 3D arecontinuously exposed to the pressure fluid. The spring 3d limits themovement of the valve piston 30 toward the left and the closed righthand end of the valve bore 23 is provided with a screw 37 by whichmovement of the valve piston 30 toward the right may be adjustablylimited.

With the parts in the position shown in Fig. l and the pump lilcontinuously driven, pressure fluid delivered by said pump into thedischarge conduit passes therethrough to the driven meinber icl which isactuated thereby. The parts will remain in the position as shownwhenever the output of the pump li) is such that it produces a pressuredrop across the orifice lll oi such an amount that the force exertedupon the valve piston 3D by iiuid from the inlet side of the orice l0equals the combined opposing forces exerted by the spring 34 and thepressure fluid from the outlet side of said orifice lll). With theviscosity of the fluid constant or neglected for purposes of thisexplanation, it will be understood that the pressure drop across theoriiice 4U (for any setting or adjustment of the size of said oririce 40and of the compression of the spring 34) will depend entirely upon therate of fluid flow therethrough, and hence will d-epend entirely uponthe output of the pump l0. It will also be understood that the valvepiston 30 will accordingly remain in its neutral position of Fig. 1 aslong as the output of the pump I0 remains constant at an amountproducing the pressure drop reservoir l2. outer end thereof willimmediately move the adacross the orifice 4t required to balance theforce exerted by the spring 34.

For example, assuming that the spring 34 exerts a force equalto 20 lbs.per sq. in. of area of the end of the valve piston III exposed to thepressure iluid, it will be seen that the valve piston 3l) will, foranyparticular setting of 4l, remain in its neutral position as shown inFig. 1 as long as the output of the pump l is constant and such that thepressure of the fluid on the inlet side of the orifice 40 exceeds thepressure of the fluid on the outlet side thereof by 20 lbs. per sq. in.

Increase in the output of the pump I0, as because of increase in itsspeed or for any other reason, will of course produce an increase in thepressure drop across the orifice 40 so that, to continue with thefigures in the above example, the pressure on the inlet side of theorice 4U will exceed the pressure of the iluid on its outlet side by anamount greater than 20 lbs. per sq. in.

below. the amount at which the corresponding i pressure drop across theorice it produces bal- This increase in relative pressures wil beimmediately communicated to the ends of the valve bore 26 and the forceexerted upon the valve piston 30 by the `fluid from the inlet side ofthe oriiice 40 will then be greater than the combined opposing forcesexerted by the spring 34 and the pressure fluid from the outlet side ofsaid orifice tu. The valve piston 30 will thus be immediately displacedfrom its neutral position and moved toward the left as viewed in Fig. 1.This movement of the valve piston 30 connects the cylinder port 28 withthe right hand end of the valve bore 26, permitting pressure uid to passfrom said right hand end of said valve bore 26 to the outer end of theadjusting cylinderll; the cylinder port 2l is simultaneously connectedwith the portion of the valve bore 26 surrounding the valve pistonsreduced portion 33, permitting exhaust of fluid from the inner end ofthe adjusting cylinder 2| into this portion of the valve bore and thencethrough the passage 29 to the Pressure fluid acting upon the justingpiston 2li in an inward or delivery-decreasing direction, thus reducingthe output of the pump l0.

The pressure drop across the orifice 40 will, of course, decreaseconformably with and immediately upon decrease in the output of the pumpit Responsive to this decrease in pressure drop, the valve piston 30will be correspondingly and simultaneously moved toward the right asviewed in Fig. 1. Decrease in the output of the pump it, accompanied bycorresponding `movement of the valve piston 30 toward the right, willcontinue until the delivered volume is reduced to the exact amount`producing the pressure drop across the orifice 4U as established by thespring t4, when the valve piston 3l will be restored to its neutralposition and will again render the adjusting piston 20 inoperative. Inother words, to again continue with the figures of the foregoingexample, the adjusting piston 20 will move the adjusting rod I1 todecrease the output of the pump I0 until said output is reduced to theexact amount producing a pressure drop of 20 lbs. per sq. in. across theorifice 40, when the valve piston 30 will be restored to its neutralposition and will again cut off communication between the cylinder ports21 and 28 and the valve bore 26.

The operation of the mechanism is, of course, the reverse of that aboveexplained 'when the output of the pump In is for any reason decreasedance oi' the forces acting upon the valve piston 30 in its neutralposition. That is to say. when the pressure drop across the oriiice 40falls below 20 lbs. per. sq. in. to further continue the figures of theforegoing example, the valve piston 30 will be displaced from itsneutral position and moved toward the right as viewed in Fig. 1. Thismovement of the valve piston 30 will admit pressure fluid from the lefthand end of the valve bore 26 to the inner end of the adjusting cylinder2| and will permit exhaust of fluid from the outer end thereof, theadjusting piston 20 and adjusting rod I1 being thus moved in an outwardor delivery-increasing direction. This movement of the adjusting piston2li and adjusting rod I1 will continue until the output of the pump I0has been restored to the amount producing the pressure drop across theorifice as determined by the spring 34, that is, until the pressure dropacross the orifice 40 is again restored to 20 lbs. per sq. in. in thecase of the above example, when the valve piston 30 will again berestored to its neutral position.

These corrective changes in thepump's output and restoring movements ofthe valve piston 30 take place almost instantaneously, and theadjustments are such as to set the corrective mechanisms into operationupon slight changes in the output to be maintained.

It will thus be seen that the spring 34 determines the amount of thepressure drop to be maintained across the orifice it, the valve piston3U being moved in one direction or the other to effect correstive changein the pumps output whenever the pressure drop across this orifice iseither greater or less than that corresponding to the force exerted uponthe valve piston 3U in its neutral position by the spring 34. Positionand movement of the valve piston 30 are determined and effected entirelyby relative pressures existing on the inlet and' outlet sidesrespectively of the orifice 40 and are independent of absolutepressures; that is to say, the valve pistons movement and positionresult from the difference between these pressures regardless of theiractual amounts.

From the foregoing it will be understood that there is only one rate offluid flow through the orifice 40 (viscosity of the iluid beingneglected or considered as constant) that'will produce a predeterminedpressure drop across said orifice 40 for any size or extent of openingthereof, and hence there is only one output of the pump l@ meeting thisrequirement. It will therefore be seen that thecontrolmechanism of thepresent invention functions to provide a predetermined output of thepump IU for each particular setting or adjustment of the spring 34 andof the orice 40, instantaneously altering the output of said pump tocorrect for any variation from this predetermined output. The output ofthe pump l0 is thus held substantially constant regardless of variationsin operating conditions as long as the setting or adjustment of thespring 34 and the orifice 40 remain unchanged. Moreover, this holds truefor any adjustment of the orifice 40 and there is an instant response toany such adjustment to change the pump delivery according- 1y whichchanged delivery is then maintained constant until a further variationin orice is made.

The corrective or compensating adjustment of output, whereby said outputis instantly adjusted to correspond to the setting of 4l and is heldsubstantially constant at that value, represents a distinct advance inhuid delivery controls for variable delivery pumps in systems of thischaracter. Such pumps as usually employed are. subject to operatingconditions which change from time to time during operation and whichaiect the fluid volume actually delivered. The speed of the drivingelement of the pump is one of these variable factors. Pumps of thisgeneral class are in most instances driven by electric motors, thespeeds of which change noticeably with the load, so that the speed of apump driven thereby may, and usually will, vary during its operation. Insuch installations the diierence between maximum speed under light loadand the minimum speed under heavy load is irequently as much as 5% ofthe maximum speed. This variation in speed will reflect a correspondingvariation in output when conventional capacity regulating means areemployed, whereas the output control of the present invention acts toimmediately correct for such speed variations.

Leakage or slip of iiuid in the pump is another factor affecting theactual iiuid volume delivered as it is physically impossible toconstruct a pump in which no such internal leakage occurs at highoperatingA pressures such as are usually employed with this type ofdevice. The amount of leakage will in turn vary with several factors Ysuch, for example, as the pressure and the temperature of the circulatediiuid, and these are subject to change during the course of ithe pumpsoperation. The output control of the present invention will, of course,act to correct for any and all variations in output of this character.

Thus, the control mechanism of the present invention holds the outputsubstantially constant, within the limits of the pumps capacity, for anyparticular setting or adjustment of the size or extent of opening of theorifice 80 and of compression of the spring 34, and acts to immediatelycorrect or compensate for variations in output due to any causewhatsoever.

Another advantage of the output control of the present invention is thatthe same control mechanism may be used Without change therein inconnection with pumps of varying.sizes or types. It does not require aseparate or special control mechanism made for or adjusted to thepeculiarities of each and every size or type of pump, as is required,for example, in conventional -controls which operate only to regulatethe iuid volume delivered per unit of movement of the pumps drivingelement without respect to any of the other factors encountered inactual operation. This is due to the fact that the control mechanism oithe present invention functions responsive to actual output and henceits action is independent of the structure, type, size, etc. of the pumpwith which it is employed.

The output of the pump I0 is preferably regulated by varying the size ofthe orifice 40, as change in the size of said orice will, of course,change the volume of fluid required to pass therethrough in order toproduce the pressure drop as established by the spring 34. An infinitelyvariable orice, such as here provided, thus makes it possible toinfinitely vary the pumps output to any desired extent 'from maximum toa minimum, such as zero.

In this connection, still another advantage of the present inventionshould be noted, that is to say, absolutely no fluid may pass to thedriven aeaspei member when the control mechanism is put in its zerooutput position by completely closing the orifice d. Much difficulty hasbeen experienced in. the past in obtaining absolute zero output fromvariable delivery pumps, particularly when their delivery is adjusted bypower means. The result of this is that there has frequently been atendency toward creeping movement of the driven member, even whenconventional control mechanisms are adjusted to their theoretical orsupposed zero output positions. This tendency toward creeping movementof the driven member is completely overcome by the output controlof thepresent invention as it will be clear that no iluid may pass to thedriven member whenever the variable orifice l is completely closed.

While it is usually preferable to adjust the output of the pump i@ byvarying the size of the oriiice lll, it is also possible to adjust itsoutput by changing the amount of the pressure drop to be maintainedacross the orifice G0, as by altering the compression of the spring 3Qby adjustment of the screw titi. Modification of the amount of thepressure drop to be maintained will correspondingly modify the uidvolume required to produce it (that is, the modified pressure drop) inpassing through an oriiice of constant size, and the control mechanismof the present invention will act to alter the pumps output accordingly.rThis arrangement may be preferred in certain installations such, forexample, as those in which the desired variation in output is small, orin which the valve housing 25 is located more accessibly than theorifice 4D, or in which it is more convenient to modify the compressionof the spring than to vary the size of the orice. This method ofadjustment will usually be somewhat less efficient, however, than thatin which a constant or unchanged pressure drop is employed and theoutput varied by change in the size of the orice, particularly when theoutput is to be varied over a relatively wide range. It will beunderstood, however, that both methods of adjustment may be employedtogether as well as being capable of separate use.

No fluid will be available for moving the adjusting piston 2Q to startdelivery of fluid by the pump lll when its output is reduced. to zero.The inner or piston rod end of the adjusting cylinder 2l is accordinglyprovided with a spring 3d, one end of which acts against the adjacentend of the adjusting piston 2l) and the other end of which acts againstthe cover 22 which closes the end of the adjusting cylinder 2i. Thearrangement is such that the spring 39 is compressed as the adjustingpiston 2G moves into its position corresponding to the Zero outputposition of the adjusting rod il. The energy thus stored up is availablefor displacing the adjusting piston 20 and adjusting rod il from thezero output position so that the pump may be caused to start delivery ofpressure uid when the variable orifice l0 is opened.

The embodiment illustrated in Fig. la diiers from that of Fig. lprincipally in the arrangement for actuating the adjusting piston 20 andfor controlling its movement. In this instance the inner or piston rodend of the adjusting cylinder 2| is continuously connected with theportion I3 of the discharge conduit, as by the passage 45, and movementand position of the adjusting piston 20 are controlled by admission ofpressure uid to or exhaust of uid from the outer end of said adjustingcylinder. The valve mechanism is accordingly modied to provide a.

single annular cylinder port 46 which is adapted to be covered by thehead 32 of the valve piston when said valve piston is in its neutralposition in which it is shown in Fig. 1a. The ends of the valve bore areconnected with the inlet and outlet sides of the orifice 4I) by thepassages 42 and 43 respectively in the same manner as shown in Fig. 1,and the left hand end of the valve bore 26 is similarly provided with aspring 34 which exerts a force upon the valve piston 30 tending to moveit toward the right.

Movement and operation of the valve piston 30 of Fig la are effectedresponsive to variation in the amount of actual pressure drop across theorifice with respect to a predetermined pressure drop there-across, inthe same manner as explained in connection with the embodiment ofFig. 1. When the valve piston 30 of Fig. 1a is displaced from itsneutral position and moved toward the left thereof, pressure fluid will'be admitted to the outer end of the adjusting cylinder 2I which will acton the end of lthe adjusting piston 2D exposed thereto, causing saidadjusting piston 20 to move inward or toward the right due to thediierential or reduced area of the other side of said adjusting piston20 produced by the piston rod. Similarly, movement of 'the valve piston30 of Fig. 1a to the right of its neutral position permits exhaust ofpressure fluid from the outer end of the adjusting cylinder 2|, so thatthe adjusting piston 20 is moved outward or toward the left by theaction of the pressure uid on the other or piston rod side thereof. TheIthrust in both directions of the adjusting piston 20 may be made equalby making the piston rod of such side that its cross sectional area isonehalf that of the adjusting piston 20, i. e., when pressure isadmitted from the source through both conduits and 24 the effective areaof the piston tending `to move Ithe same to the right is equal to thearea of the rod and when the'conduit 24 is connected with the exhaustand the conduit 45 to the source of pressure, the effective area of thepiston tending to move the same to the left is equal to the piston areaminus the rod area and if the rod area is half the area of the piston,then the thrust in opposite directions for the same unit pressure willbe equal,

The embodiments illustrated in Figs. 1 and 1a are particularly suitablefor use with a metering orifice which is positioned in lthe fluidcircuit at a point intermediate the pump and the driven member. This isthe case because these embodiments pre-suppose that the pressures of theuid admitted to the ends of the valve bore are sufficient to actuate theadjusting piston. The invention may be modified, however, for use with avariable orificel which is positioned at any point in the dischargeconduit, as for example, the embodiment shown in Fig. 2 whichillustrates the output control of the present invention as employed inconnection with a variable orifice located in the portion of the fluidcircuit intermediate the driven member and the reservoir.

The valve piston (Fig. 2) is provided with four heads 5I, 52, 53 and 54respectively, separated by reduced portions 55, 56, and 51. The ends ofthe valve bore 25 are supplied with fluid from the portion I5 of thedischarge conduit on the inlet and outlet sides respectively of thevariable orifice 40, as by the passages 42 and 43, and the left hand endof the valve bore 26 is Iprovided with a spring 34 which exerts a. forceupon the'valve piston 5I) tending to move it toward the right as viewedin Fig. 2, in the same manner as explained in connections with theembodiment of Fig. 1. K

'Ihe pressure of the fluid on both the inlet and outlet sides of theoriilce' 40 are insufficient to actuate the adjusting piston 20 andmeans are accordingly provided for supplying pressure iluid for thispurpose from another source. According to the arrangement of Fig. 2,pressure Huid for actuation of the adjusting piston 20 is suppliedthrough a suitably connected passage 58 to the portion of the valve bore2B surrounding the valve pistons reduced portion 56. This pressure fluidmay be taken from any suitable source, such as an auxiliary pump, or, ashere shown, from the portion I3 of the discharge conduit, that is, theportion of the discharge conduit intermediate .the pump I Il and thedriven member I4. The portions of the valve bore 26- surrounding thevalve pistons reduced portions 55 and 51 are appropriately connectedwith the exhaust passage 23 leading to the reservoir I2.

It will be seen from the foregoing that movement of the valve piston 50responsive to varia` tion in pressure drop across the orifice 40, withrespect to the predetermined pressure drop thereacross as determined bythe spring 34, takes place in the same manner as explained in-connection with the embodiment of Fig. l. Inasmuch as the pressurefluid for actuating the adjusting piston 20 is supplied intermediate theheads 52 and"53 of the valve piston 50, it will also be seen that thefluid connections between the cylinder ports 21 and 28' and the ends ofthe adjusting cylinder 2l must be the reverse of those shown in Fig. 1;that is to say, the pont 21' connects with the passage 24' leading toand connecting with the outer end of the adjusting cylinder 2 I, whereasthe port 28 connects with the passage 23' connecting with the inner end'thereof The operation oi' the mechanism is otherwise the same as thatexplained in connection with Ithe embodiment of Fig. 1 and functions tocause a substantially constant output of fthe pump I0 for any particularsetting or adjustment of the size of the orice 40 and of the compressionof the spring 34.

The embodiment illustrated in Fig, 2 will work equally well, and in thesame manner, if the variable orifice 4U is positioned in the pontion I3of the discharge conduit and the passages 42 and 43 are correspondinglyconnected with said portion I3 on the inlet and outlet sides of saidoriilce 40. The point in the discharge conduit at which the orifice 40is located affects only fthe absolute or actual pressures of the fluidadmitted to the ends of the valve bore 26 and does not affect therelative pressures thereof, that is, rthe pressure drop across theorifice 4D. As already explained, functioning of the valve pistondepends entirely upon the amount of the pressure drop or difference inpressures existing on the inlet and outlet sides of fthe orifice 4U andis otherwise independent of the actual amounts of said pressures. Thearrangement of Fig. 2 may also be modified. vas already stated, byconnecting the passage 5B with a separate auxiliary pump or other sourceof pressure fluid instead of taking this fluid from the portion I3 ofthe discharge conduit as shown. This may prove advantageous ininstallations in which the pressure of the fluid in vthe dischargeconduit I3 is insumcient for actuating the adjusting piston 20.

The embodiments of the present invention which have been described up tothis point will, as explained, act to correct or compensate for anyvariation in the output of the pump I0 with respect to the outputdetermined by the setting or adjustment oi' the spring 34 and orifice 4Uand will thus hold the delivered volume substantially constant providedthe viscosity of the circulated fluid is likewise constant. 'I'here aresome instances in which change in viscosity of the fluid may beneglected and in which arrangements of the general character of theembodiments described will meet all practical requirements. For example,in some installations there may be relatively little change in operatingtemperatures, and hence in viscosity of the fluid, as in instances wherethe temperature of the circulated fluid is regulated by suitable heatexchangers or the like. There are also other types of installations inwhich the temperature of the iluid will rise, upon starting theoperation of the device which the fluid actuates, but will soon reach apoint at which it will be substantially stabilized and at which it willremain throughout the continued operation of the device. In the lattercase it is sometimes practical to provide adjustment or setting of theorice 40 and spring 34 adapted to give the desired output at thestabilized temperature, since the relatively smaller. output at thelower temperatures encountered initially may be acceptably used duringthe period prior to temperature stabilization.

The output of the pump I0 for any particular setting or adjustment ofthe spring 34 and oriilce 46 will, however, be altered upon change inviscosity of the circulated fluid unless means are provided tocompensate for such viscosity change. Resistance to flow through theorifice will decrease as the viscosity of the iluid decreases and,similarly, will increase upon increase in the ilulds viscosity,producing corresponding changes in the pressure drop across the oriiiceif the fluid passes therethrough at a constant rate of flow. Movement ofthe valve pist'on occurs responsive to variation in actual amount ofpressure drop across the oriiice with respect to a predetermined amountthereof, and this is the case irrespective of the cause of saidvariation. Change in amount of the pressure drop due to viscosity changewould, therefore, result in movement of the valve piston to alter theoutput of the pump I6, and this would take place without any change inthe adjustment of either the spring 34 or the orifice 40. In otherWords, without viscosity compensating means, decrease in viscosity ofthe fluid will produce a decrease in the pressure drop across theorifice 46 resulting from a constant output of the pump i6 and the valvepiston will automatically move to cause the output of the pump I 0 to beincreased to the output at which the pressure drop is again restored tothe amount determined by the spring 34. It will thus be seen, therefore,that the output of the pump I IJ, for any particular adjustment of thespring 34 and the orice 46, will vary in accordance with variation lnthe viscosity of the fluid unless means are provided to compensate forsuch viscosity variation.

This variation in output of the pump I0 due to uncompensated viscositychange may cover a wide range, the exact amount of which will dependupon the range of temperatures encountered during operation and theviscosity characteristics of the particular fluid employed. Oil is thefluid usually used in systems of this general character and, as is wellknown, is subject to relatively large variation in viscosity in therange of operating temperatures usually encountered in practice. Forexample, one lubrieating oil particularly well suited for use as thecirculated fluid in these systems, and at present regarded as notablebecause its viscosity varies much less than that of the average oil, issaid to have a viscosity of approximately 220 S. S. U. (Saybolt SecondsUniversal) at 80 F., a viscosity of 150 S. S. U. at 100 F., and aviscosity of approximately 66 S. S. U. at 160 F. These figures indicatethe wide variation in viscosity frequently encountered in practice and,unless compensated for, the output of the pump i0 will likewise varycorrespondingly as already explained. Means are accordingly provided tocompensate for change in viscosity of the circulated iluid in order thatthe output of the pump I0 will be unaiected thereby, one embodiment ofwhich is illustrated in Fig. 3 and will now be described.

The hydrostatic fluid pressure system and arrangement illustrated inFig. 3 is generally similar to the embodiment of Fig. 1 except for theprovision of the viscosity compensating mechanism. The valve mechanismincludes a valve piston 30 slidable within the valve bore 26 andprovided with a pair of heads 3l and 32 which are adapted to cover theannular cylinder ports 21 and 28 respectively when said valve piston isin its neutral position in which it is shown in Fig. 3. The valve piston30 also includes a pair of extension rods 6l and 62, of equal diameter,which extend from the heads 3| and 32 respectively and project throughsuitable openings in the closures of the ends of the valve bore 26 insuch manner that they form substantiailly iluidtight sliding tstherewith. The ends of the adjusting cylinder 2| are connected with thecylinder ports 2l and 28 respectively by the passages 23 and 24 and theportions of the valve bore 26 surrounding the valve rods 6| and 62respectively are connected with the inlet and outlet sides respectivelyof the orifice 40 by the passages 42 and 43 which are shown asconstricted (42' and 43') at the valve bore 26 in order to reduce anytendency toward hunting or "chatter. The spring 34 in the left hand endof the valve bore 26 surrounds the valve rod 6| and exerts.

a force upon the valve piston 30 tending to move it toward the right, asin the embodiment of Fig. 1. It will thus be seen that this portion ofthe valve mechanism is substantially identical with that of theembodiment of Fig. 1 from which it differs, as described up to thispoint, only with respect to the provision of the valve rods 6| and 62.This portion of the valve mechanism is therefore capable of operation inthe same manner as that described in connection with Fig. 1 and may, ifdesired, be so employed without regard to the viscosity compensatingmechanism which will now be described.

The viscosity compensating mechanism includes a pair of cylinders orbores 63 and 64 respectively, for convenience termed compensatingcylinders, which in the present instance are formed in the members 59and 60 which close the ends of the valve bore 26. The compensatingcylinders are provided with slidably iitted pistons, termed"compensating pistons, operatively connected With the valve piston 30and the ends of the rods 6| and 62 are utilized as the compensatingpistons in the present embodiment. Each compensating piston is of suchsize that its cross-sectional area equals the crosssectional area of theportions of the valve piston 30 which are exposed to the pressure iluidin the ends of the valve bore 26: this relation is here obtained bymaking the rods 6I and 62 of such size that the cross-sectional area ofeach of them is one-half the area of a section through the heads 3| or32 of the valve piston 30.

The viscosity compensating mechanism also includes an' auxiliary fluidcircuit, which may be termed the compensating circuit," which in turnincludes a small, constant capacity pump 61 adapted to be continuouslydriven at a constant speed. The pump 61 receives its supply of oil orother fluid from the reservoir l2 and in the present instance is shownas provided with an inlet conduit 68 which connects with the inletconduit il of the main pump lli. It will thus be seen that the fiuidsupplied to the pump 61 will at all times be of exactly the sameviscosity as the fluid supplied to the main pump I 0, and

hence of the same viscosity as that of the fluid passing through theorifice 40. The pump 61 is also provided with a discharge conduit 69leading to the reservoir l2 and having an orifice 10 which is shown as avariable orifice although a fixed orifice may be employed since the sizethereof is not adjusted during operation.

With this arrangement, the amount of the pressure drop across theorifice 'l0 will be constant as long as the viscosity ofthe circulatedfluid is constant but will vary immediately upon and conformably withany change taking place in the viscosity of said fluid. As the uldpassing through the orifice 10 is of substantially the same viscosity asthe fluid passing through the orifice 40, it will be seen that change inviscosity of the circulated fluid will produce identical changes in theamounts of the pressure drops across both of these orifices for constantrates of fluid flow through them. The rate of fluid flow through theorifice 10 is constant for the reason that the pump 6l is-of constantcapacity and is driven at l a constant speed. Hence the change inpressure drop across the orifice lil is an exact measure of thecorresponding change which takes place during the same interval in theamount of the pressure drop across the orifice 40 for any particularoutput of the pump I; that is to say, it is an exact measure of theeffect of the change in viscosity upon the amount of the pressure dropacross the orifice 40 produced by a constant output of the pump I0 andwith the adjustment of the orifice 40 and the spring 34 unchanged. Byway of illustration and further explanation. with the arrangement asabove described, if change in viscosity of the fluid results in adecrease of lbs. per sq. in. in the amount of the pressure drop acrossthe orifice '70, this viscosity change will also decrease the amount ofthe pressure drop across the orifice 40 by 5 lbs. per sq. in. if theoutput of the pump I0 remains constant and no change is made in thesetting or adjustment of the orifice 40 and of the spring 34.

According to the preferred embodiment as illustrated in Fig. 3, thechange taking place in the amount of the pressure drop across theorifice 'lil is employed to correspondingly modify the amount of thepressure drop to be maintained across the orifice 40. The compensatingcylinder 63 is therefore connected. as by a passage 65, with thedischarge conduit 69 at a point on the inlet sicle of the orince 1|) andthe compensating cylinder 64 is similarly connected, as by the passage66 with said discharge conduit 69 at a point on the outlet side of saidorifice 10. The compensating cylinders 63 and 64 are thus supplied withfluid having the same pressures as the pressures existing on the inletand outlet sides respectively -of the orifice 10.

Two additional opposing forces are' thus brought to bear upon the valvepiston 30 by the compensating pistons. These two opposing forces have anet difference or resultant tending to move the valve piston 3U towardthe right, this net difference or resultant corresponding to and varyingwith the amount of the pressure drop existing across the orifice 10. Thenet effect, therefore, is that of a force tending to move the valvepiston 30 toward the right and which varies conformably with the amountof the pressure drop across the orifice 10, and hence likewise variesconformably with the viscosity of the fluid.

The net difference or resultant of forces thus exerted upon the valvepiston 30 by the compensating pistons combines with the`force exerted bythe spring 34 to determine the amount of the pressure drop to bemaintained across the orifice 4II which is, therefore, modified bychange in the amount of said net difference or resultant. The amount ofthe pressure drop to be maintained across the orifice 40 is thusmodified conformably with the change in viscosity of the fluid and inexact accordance with the change occurring in the amount of the pressuredrop across the orifice 40, with a constant rate of fluid flowtherethrough, resulting from viscosity change. In other words, decreasein viscosity of the uid reduces the amount of the pressure drop acrossthe orifice 40 produced by a constant output of the pump I0 andtherefore reduces the net difference or resultant of the forces exerted-upon the valve piston 30 by fluid from the inlet and outlet sides ofthe orifice 4U, which net difference .or resultant tends to move thevalve piston 33 toward the left; this decrease in viscosity simulftaneousl'y also reduces, by exactly the same amount, the sum of thecombined opposing forces which tend to move the valve piston 30 towardthe right. In the same manner, increase in viscosity of the fluidsimultaneously and equally increases the 'forces tending to move thevalve piston toward the right and those tending to move it toward theleft. The relative balance of forces acting upon the valve piston 30 istherefore undisturbed by change in Viscosity of the fiuid.

Change in viscosity of the circulated duid thus merely affects theamount of the pressure drop to be maintained across the orifice 40, andalters the amount thereof in exact accordance with the effect of suchchange in viscosity upon the amount of the pressure drop actually takingplace across the orifice 43 with the output of the pump I0 constant andwith the adjustment of the orlce 40 and of the spring 34 unchanged.

The compensating mechanism therefore c0- operates with the other partsof the output control of the present invention to hold the deliveredvolume of the pump substantially constant for any setting or adjustmentof the orifice 4U and spring 34, regardless of change in viscosity,speed or operation of the pump IIJ, leakage or for any other reasonwhatsoever.

In order to prevent leakage of fluid from the valve bore 26 into thecompensating cylinder 63 and 64, which leakage might possibly affect thepressures existing in said compensating cylinders, the members 59 and 60are provided With counterbores or leakage grooves li and 'I2respectively, intermediate the ends of the valve bore 26 and the exposedends of the compensating pistons, and said leakage grooves areappropriately connected with the exhaust passage 29 which leads to thereservoir I2.

The pump 61 may be of very small capacity as it is only necessary for itto circulate suicient fluid to produce the required pressure drop acrossthe orice 10. The actual volume circulated by the pump 61 is otherwiseunimportant and the adjustable feature of the orifice makes it possibleto obtain this required pressure drop with an extremely small output ofsaid pump 61. The principal requirement is that the pump 61 be driven ata substantially constant speed in order that its output may besubstantially constant; this may be readily accomplished as the load issmall and is also substantially constant.

The pressure of the delivered fluid of the pump 61 will always berelatively low so that an inexpensive type of pump may be employed andpower losses incident to its operation will be small. The use of thepump 61 provides a simple and effective means for obtaining a constantrate of fluid flow through the orifice 'I0 but it will be understood,however, that any other means for obtaining said constant rate of fluidflow there-through may be substituted for` the arrangement as shown.

The compensating mechanism as illustrated in Fig. 3 and as abovedescribed has numerous advantages. The most important, of course, isthat compensating action is in exact accordance with the effect of thechange in viscosity actually taking place, and hence is absolutelyaccurate. A further advantage is that it permits the use of the samecompensating mechanism with almost any type of fluid and range ofoperating temperatures instead of requiring mechanism designed orprecisely adjusted to fit the peculiar viscosity characteristics of eachindividual uid in each range of operating temperatures. The importanceof this will be understood from again considering the fact that almostevery lubricating oil, which is the uid usually employed, has its ownpeculiar viscosity characteristics. For example, the oil to whichreference has been made heretofore is said to have, as previouslystated, a viscosity of approximately 220 S. S. U. at 80 F., a viscosityof 150 S. S. U. at 100 F., and a viscosity of approximately 66 1S. S. U.at 160 F., whereas two other oils which may be acceptably used in someinstances are said to have viscosities of approximately 400 and 675 S.S. U. respectively at 80 F., viscosities of approximately 260 and 370 S.S. U. respectively at 100 F. while the viscosity of both of them is saidto be about 100 S. S. U. at 160 F. These figures indicate the f greatadvantage of viscosity compensating mechanism that is universal in thesense that it may be employed and will function accurately withpractically any fluid and in practically any range of temperatures, andin which no precise adjustment is required. A further advantage ofmaking the orifice 10 of the variable type is also seen in thisconnection, as it permits adjustment to produce sufiicient pressure dropthere-across to provide full viscosity compensating modification in theamount of the pressure drop to be maintained across the orice 40 withoutmaking the amount of the pressure drop across the oriiice 'l0unnecessarily large.

Fig. 4 illustrates a hydrostatic fluid pressure system embodying furthermodifications of the present invention. This figure is included incopending applications Serial Numbers 250,864 and 268,253 in whichcertain parts of the subject matter thereof are claimed. In thisinstance the variable delivery pump ||0 is shown asI having a pair oflaterally movable delivery-determining elements or adjusting rods Illsuch as fourid, for example, in variable capacity vane pumps of thedouble-acting type, one of which is disclosed in U. S. Patent No.2,141,170. Pumps of this general character are in reality "double pumpscomprising two "pumping sections, and the capacity of each pumpingsection" may be varied independent of variation in the capacity of theother "pumping section thereof. The pump |I0 is therefore the equivalentof two smaller variable delivery pumps, and two separate smaller pumps,similar to the pump I0 of Fig. 1 for example, may be substitutedtherefor lf preferred.

Each adjusting rod |I1 of Fig. 4 is independently movable to control aportion of the pumps total output, that is, to control the output of itspumping section, exactly as would be the case if each of said adjustingrods controlled the output of a separate pump. For example, if theoutput of the pump H0 at a particular speed is 20 gals. per min., eachadjusting rod ||1 would be,independently movable to independentlycontrol he output of 10 gals. per min. if the total outpu is equallydivided between the two pumping sections as is usually preferable,although not necessarily the case. 'I'he pump I I0 delivers its maximumoutput only when both adjusting rods are in their extreme outwardposition, with respect to the center of the pump H0, and similarly saidpump ||0 delivers its minimum output when both of said adjusting rods||1 are in their extreme inward position. It will thus be understoodthat the positions of the adjusting rods determine the output of thepump H0.

Since the adjusting rods Ill are independently movable, it will also beunderstood that there is an almost infinite number of combinations ofrelative positions of said adjusting rods H1 capable of producing eachoutput of the pump ||0 intermediate its maximum and minimum outputs. Forexample, to continue the gures of the foregoing example, a delivery of10 gals. per min. by the pump ||0 would be produced by moving bothadjusting rods so as to cause the output of each of the two pumpingsections" to be 5 gals. per min.; or said delivery of 10 gals. per min.could be produced by moving one adjusting rod to its extreme inwardposition so that the delivery of its pumping section is zero and movingthe other of said adjusting rods to its extreme outward position so thatthe delivery of its pumping section is 10 gals. per min.; or saiddelivery of 10 gals. per min. could be produced by making the positionsof the two adjusting rods Ill such that said total output of 10 gals.per min. is divided between the two pumping sections in any otherproportion. The output control of the present invention functionsaccurately irrespective of the division of the total output between thetwo pumping sections, as will be more fully explained.

The adjusting rods I l are attached, as indicated at H8, to the ends ofthe piston rods of a pair of adjusting pistons |20 by which theirmovement and position are controlled. The adjusting pistons |20 arereciprocable in adjusting cylinders |2| and are adapted to be movedoutward by springs |38 which b ear against the adjacent faces of saidadjusting pistons 20 and against the covers |22 which close theadjusting cylinders |2l. Movement and position of the adjusting pistons|20 are controlled by admission oi pressure fluid to or by the extent ofthe exhaust of fluid permitted from the outer ends of the adjustingcylinders I2I. The outer ends of the adjusting cylinders |2| areaccordingly connected with the ends of a branched passage |24 leading toand connecting with the single annular cylinder port |21 in; the valvebore |28. It will be understood that the springs move the adjusting`pistons |20 and their attached adjusting rods ||1 in an outward ordeliveryincreasing direction as far and as rapidly 1as permitted by theexhaust of fluid from the outer ends of the adjusting cylinders |2I. Itwill also be understood that admission of pressure iiuid to the outerends of the adjusting cylinders |2| will move the adjusting pistons |20and adjusting rods ||1 in an inward or delivery-decreasing direction,the springs |38 being further compressed upon such inward movement. Inorder to permit the escape of any fluid leaking past the adjustingpistons |20, the inner ends of the adjusting cylinders |2| are connectedwith the ends of a branched passage |23 which is suitably connected withthe reservoir I 2, as by a connection with the discharge conduit 88 at apoint on the outlet side of the orifice 10.

The valve mechanism illustrated in Fig. 4 is generally similar to thatshown in Fig. 2, although differing therefrom in certain particulars aswill be presently explained. It includes a valve piston slldably fittedwithin the valve bore |28 of the valve housing |25 and having threeheads |5|, |52 and |53 respectively, separated by the reduced portionsand |56. 'I'he head 52 covers the cylinder port |21 when the valvepiston |50 is in its neutral position in which it is shown in Fig. 4.Movement of the valve piston |50 to the right of its neutral positionconnects the cylinder port |21 with the portion of the valve boresurrounding the valve pistons reduced portion |55. so that fluid may beexhausted from the outer ends of the adjusting cylinders |2|, suchexhausted fluid returning to the reservoir |2 through the passage 29. Asalready explained, the adjusting pistons move outward under theinfluence of the springs |38 when the outer ends of the adjustingcylinders |2I are connected with the exhaust.

Movement of the valve piston |50 to the left of its neutral positionconnections the cylinder port 21 with the portion of the valve bore |26vsurrounding the valve plstons reduced portion |58: pressure fluidsupplied to this portion of the valve bore |28, as through the passage58, may then pass around the reduced portion |56, through the cylinderport |21 and the branched passage |24 to the outer ends of the adjustingcylinders 2|, causing the adjusting pistons |20 to move inward. Thevalve head |52 is shown as provided with a number of conventional V-notches on its end adjacent the reduced portion |56, these notchesproviding a relatively gradual connection between the reduced portion|56 and the cylinder port |21 when the valve piston moves to the left ofits neutral position, thus reducing any tendency toward abrupt movementof the adjusting pistons |20.

In some instances it has been found desirable to provide a centeringmechanism for the valve piston to assure its proper location withrespect to the other parts when said valve piston is in its neutralposition and for the further purpose of reducing or eliminating any-tendency toward hunting or surging that may be present. A centeringmechanism suitable for this purpose and of conventional type isillustrated in Fig. 4, which may likewise be employed with theembodiments heretofore described.

As shown in Fig. 4, the centering mechanism is located in the bore of ahousing which is suitably attached to the right hand end of the valvehousing |25, as by threaded engagement therewith. The outer end of thehousing 80 is in turn closed by an auxiliary valve housing 80, thepurpose of which will be later explained, and the two parts areappropriately fastened together, as by the screws 82. It is preferablethat there be substantially no pressure oi any fiuid that may enter thebore of the housing 80, as through leakage, and a passage 93 leading to.the reser- -voir I2 is accordingly connected with said bore.

The centering mechanism also includes a pair of washers or annularmembers 8| and 82\freely slidable upon a rod 83 which is of smallerdiameter than that of the valve piston |50 to which it is securelyattached as by the pin 84. A relatively light compression spring 85 ispositioned between the Itwo washers 8| and 82 and urges them toward thestops or seats 81 and 88 formed respectively by the end of the valvehousing |25 and the reduced end portion of the bore of the housing 80.The arrangement is such that when the valve piston |50 is in its neutralposition the washer 8| is in contact both with its seat 81 and with theadjacent end of the valve piston |50; and similarly, the washer 82 isthen in contact with its seat 88 and is also in contact with theadjacent end of the auxiliary valve piston |54 which is carried by therod 83 and ispositioned with respect thereto by the nut 88 on thethreaded end of said rod 83.

The washers 8| and 82 may thus move .toward each other, upon furthercompression of the spring 85, and one or the other of them is so movedwhenever the valve piston |50 moves out of its neutral position.- Inother words, when the valve piston |50 moves toward the right of itsneutral position lthe end of said valve piston |50 will move the washer8| toward the right and away from its seat 81; the Washer 82 remainsagainst its seat 88, the rod 83 sliding through the hole at the centerof said washer 82. Similarly, when the valve piston |50 moves to theleft of its neutral position the end of the auxiliary valve piston |54moves .the Washer 82 toward 'the left and away from its seat 88, thewasher 8| remaining against its seat 81 as the rod 83 slides lthroughthe hole at its center. In this manner Ithe centering mechanism tends.to main- .tain the valve piston |50 in its neutral position andpresents a relatively slight increased resistance to displacement ofsaid valve piston in either direction.

The modified valve mechanism of Fig. 4 also includes an auxiliary valvebore formed in the auxiliary valve housing 90 and of the same diameteras the valve bore |25, into which thev tached to the left hand end ofthe valve housing i |25 by threaded engagement therewith. Alsopositioned in the bore of the housing is the spring 34 having abutmentpieces 35 and 96 respectively on each of its ends. The abutment piece 96bears against the adjacent end of 4the valve piston |50 and the abutmentpiece 35 bears against the end ofthe screw 36 which extends through theclosed end of the housing 95 and provides means for adjusting thecompression of the spring 34 as already explained. As will be readilyunderstood, the spring 34 exerts a force upon the valve piston |50tending to move the valve piston assembly toward the right as viewed inFig. 4 and the abutment piece 90, which is of larger diameter than thatof the valve bore |20, limits the maximum distance in this direction towhich said valve piston assembly can be moved by said spring 34.

As in the embodiments previously described, position and movement of thevalve piston assembly are determined and eifected by the actual pressuredrop existing across the metering orifice 40 relative to a predeterminedpressure drop thereacross. The orifice 40 is here shown as positioned inthe portion I5 of the discharge conduit and the end of the auxiliaryvalve bore in the auxiliary valve housing 90 is accordingly connected,as by the passage 42, with said portion I5 of said discharge conduit ata point on the inlet side of the orifice 40. It has been found, however,that when the metering orifice 40 is positioned in the discharge conduitat a point intermediate the driven member I4 and the reservoir I2, thepressure of the fluid on the outlet side of the orifice 40 is so smalland subject to such minor variations that in practice it may frequentlybe neglected. In other words, the actual amount of the pressure existingon 'the inlet side of the orifice 40 may alone be practically employedin many instances as the measure of the pressure drop across saidorifice and hence may be alone employed in measuring the rate of fluidflow therethrough. The outlet side of the orifice 40 is therefore notconnec-ted with the valve mechanism in the arrangement of Fig.

4. The force exerted upon the valve piston assembly by the action of thepressure fluid from the inlet side of the orifice 40 upon the end of theauxiliary valve piston |54 is thus opposed only by the force of thespring 34 when the valve mechanism is employed without viscositycompensating means. This arrangement has the obvious advantage ofsimplifying the structure and of reducing the necessary fluidconnections.

Fig. 4 also illustrates, however, viscosity compensating mechanismdiffering from that of Fig. 3 only in the particulars which will now beexplained. In this instance fiuid from the inlet side of the orifice I0is admitted to the bore of the housing 95, which also serves as acompensating cylinder, where it acts upon the exposed end of the valvepiston |50 (or more accurately, acts upon the end of said valve piston|50 through the 'abutment piece 96) and thus supplements the forceexerted by the spring 34. As in the case of the fiuid pressure on theoutlet side of the orifice 40 of Fig. 4, lt has likewise been found thatthe vpressure on the outlet side of the oriflce l0 may be .neglected formost practical purposes. The pressure on the inlet side of the orificel0 is accordingly alone employed as the measure of the pressure dropthereacross and the valve piston assembly is therefore not acted upon byfiuid from the outlet side of said orifice l0. Compensation forviscosity change is thus effected by variation in the supplementaryforce exerted upon the valve piston by pressure uid in the bore of thehousing 95, the pressure of this fluid varying substantially with thepressure drop across the orifice 'I0 and substantially with the changein viscosity of the circulated uid. In this manner the amount of thepressure drop to be maintained across the orifice 40 is modifiedsubstantially in accordance with the effect of the change in viscosityof the fluid upon the pressure drop across the orifice 40 caused by aconstant rate of iiuid fiow therethrough.

The operation of the arrangement of Fig. 4 will be understood from theexplanations previously given, from which it will be seen that theoutput of the pump I I0 will be held substantially constant for anyparticular setting or adjustment of the size of the orifice 40 andcompression of the spring 34, regardless of change inv viscosity of thefluid, speed of operation of the pump or any other factor. Relativemovement and position of the two adjusting pistons |20 and theirattached adjusting rods II'I are not important, as the output of thepump I|0 will be held to the proper amount irrespective of the divisionof this amount between the two pumping sections. For example, if thecontrols are adjusted to provide an output of l0 gallons per minute,this output wil1 be maintained regardless of the amount contributed tothis total by each pumping section." It is immaterial, therefore,whether this output of l0 gallons per minute comprises equal outputs of5 gallons per minute under the control of each adjusting rod II1, orwhether it is comprised of an output of 6 gallons per minute under theinfluence of one adjusting rod and an output of 4 gallons per minuteunder the influence of the other adjusting rod, or whether it iscomprised of any other combination of outputs which total 10 gallons perminute. This is an important feature as it eliminates the necessity formechanica1 linkage or other mechanism that would otherwise be requireda's, for example, in a unitary control of the conventional follow-uptype adapted to control the joint outputs of a double acting" pump ortwo equivalent separate pumps.

Fig. 4 also best illustrates the relief valve feature of the presentinvention which may likewise be provided in the other embodimentspreviously described by proper proportion of the parts. When thevariable orifice 40 is completely closed, any uid delivered by the pumpIIO will immediately build up a relatively high pressure in the portionI5 of the discharge conduit on the inlet side of said orice 40. Thecorresponding force exerted upon the auxiliary valve piston |54 willimmediately move the valve piston assembly toward the left until thevalve head |52 is to the left of the exhaust passage 29. The reducedportion |56 of the valve piston is made of such length that it connectsthe point of admission of the pressure fluid from the passage 58 withthe exhaust passage 29 when the valve piston assembly is displaced tothe left to this extent. With the parts in this displaced position, itwill be seen that pressure fluid from the portion i3 of the dischargeconduit will be exhausted through the passage 58, the valve bore |26 andthe exhaust passage 29.

This "safety valve or relief valve feature makes it unnecessary toprovide absolute zero output of the pump and hence makes it unnecessaryto resort to the precise dimensions or adjustments that are usuallyrequired to provide an output of absolutely zero. It also makes itpossible to establish the minimum output of the pump at an amountslightly greaterthan aero so that pressure iluid will always beavailable for movement of the power adjusting mechanism,

as is particularly desirable in instances where o it is not practical toemploy the spring-loaded arrangement of Fig. 4 or that of Fig. l. Itwill be noted that no movement of the driven member Il occurs duringthis exhaust of fluid inasmuch as the orifice l is completely closed andprevents the exhaust oi' fluid by the driven member; Ain lfact, the onlypossible movement of the driven memberis the slight amount correspondlngto the volume of fluid necessary to move the valve piston assembly tothe `by-pasaing or exhaust position, and this is negligible. It willalso be noted that the pressure of the fluid required to move the valvepiston` assembly to the exhaust position is not excessive, as theresistance presented to movement in this direction is, in fact,relatively small. The exhaust oi' :duid from the vportion I3 of thedischarge conduit therefore takes place at a lower pressure than wouldexist ii a separate relief valve of conventional type were provided insaid portion I3, thus reducing .power losses. As already stated. this"relief valve" feature may be provided in the previously describedembodiments by proper proportioning of the parts. y

While I prefer to employ the previously described hydraulic means tmodify the pressure so that the chamber 2l! forms, in effect. anenlarged portion of said discharge conduit.

'The lower end of the rod 2H bears against a curved or flexed member 2|!which is positioned in the chamber 2li and is exposed to the circulatedfluid passing through said chamber. The member 2lil is made of amaterial having a high oo'emcient of expansion and is here schematicallyshown as a conventional bi-metallic thermostatic member. The side wallsof the chamber 2l! coniine the ends of the member 2li and prevent theirfurther spreading movement so that the midpoint of said member 2 I6 isforced upward as said member 2li expands.

' It is necessarythat the parts of the compensating mechanism behydraulically balanced to prevent their displacement by action ofunequal fluid pressures. Fluid passages are accordingly providedin thebearing plugs 202 and 2l! so that the chambers 203 and 2li and the endof the valve bore tothe left of the plunger 200 will at all timescontain iluidunder the same pressure as that on the outlet side of theorifice 40.

which is adapted to bear against the adjacent 40 end of the spring .34to determine the compresagainst the face of a cam 206 which is pivotallymounted as at 201 in the cam chamber 203.` It will thus -be seen thatmovement of the cam 2li about its pivot will alter the position of therod 20I and plunger'200, and hence will alter the compression of thespring 3Q. I'he arrangement is such that rotation of the cam 206 in aclockwise direction causes increased compression of the spring 3l andits rotation in a counter-clockwise direction causes decrease in saidcompression.

The cam 200 is adapted to be rotated respon-` sive to change inviscosity of the circulated fluid so that compression of the spring Mmay be correspondingly modified. The cam 200 is` accordingly pivotallyconnected with one end of a short link 208, the other end of which ispivotally connected with the'upper end of a rod II I` whichis slidinglysupported by a pair of bearing plugs 2i2. The lower end of the rod 2IIextends into a smaller chamber 2I5, the casings of the chambers 203 and2 I5 being suitably fastened together Vto prevent escape of fluid at thejoint between them. 'I'hechamber 2li is connected into the portion I3 ofthe discharge conduit at a point intermediate the orifice 40 and thevdriven member I4, that is, on the outlet side of the oriiice VChange inviscosity of the circulated iluid will, of course, always be accompaniedby change in temperature. A decrease in viscosity will, therefore, beaccompanied by an increase in temperature of the iluld which will becommunicated torthe member 2li andcause expansion thereof. An upwardforce will thus be exerted upon the rod 2li which, through the link 209,will correspondingly rotate the cam 205 in a counter-clockwisedirection, thus reducing the compression of the spring 34 and hencelikewise reducing the amount of the pressure drop to be maintainedacross the orice Il. Similarly, an increase in viscosity of the duidwillgbe accompanied by a decrease in its temperature which, will causecontraction of the member 2li, allowing the rod 2II to be moved downwardand the cam 206 to be correspondingly rotated in a clockwise direction;this movement place under the influence of the spring 34 a of y thetension spring 205,.one end of which is attached to the cam 200 and theother end of which is attached to the cover 204 of the cam chamber Itwill thus be seen that by proper proportion of theparts the compressionof the spring u may be modined conformably with the change inthe-viscosity of the circulated fluid, thus altering the pressure dropto be maintained across the oriilce 4l so that change in viscosity ofthe fluid will not alect the output of the pump I Il. With thisarrangement it is necessary to provide a special cam for use with eachparticular fluid, as the cam face must necessarily be shaped accordingto the peculiar viscosity characteristics of the iluid with which it isused. This arrangement has the advantage, however, of providing aself-contained compensating mechanism which does not require anauxiliary fluid circuit and it maybe used in connection with a meteringoritice positioned either intermediate the pump and driven member, asillustrated in F'ig. 5, or intermediate the driven member and thereservoir.

Compensation for change in'viscoslty of the circulated fluid may alsorbe accomplished without .modifyingthe amount of the pressure drop to bemaintained across the metering orifice but by modifying the size orextent of opening of said-metering orifice, as, for example, by thearrangement illustrated in Fig. 6. The metering oriilceof Fig.. 6 may besubstituted for the variable ermee `n' of Figs. 1, 1a, and 2. It isshown as of the conventional balanced piston valve type and includes theusual valve piston 220 slidably fitted within the bore 226 of the valvehousing 225. The dischargeconduit is connected with the valve bore 226through a pair of spaced annular ports 221 and 228 respectively and, asindicated by the arrows, fluid enters the valve bore through the port221 and passes out through the port 228.

The valve piston 220 comprises a pair of heads 22| and 222 separated bya reduced portion 223 which is preferably tapered to provide gradual 4opening and closing of the orifice and thus facilitate adjustment: Thesize of the metering orifice is varied by axial movement of the valvepiston 220 in the valve bore 226, this movement varying the extent towhich the head 22| restricts communication between the port 221 and theportion of the valve bore 226 leading to the port 228. The arrangementis such that the valve piston may be moved to any desired extent so thatthe size or extent of opening of the metering orifice may be -infinitelyvaried from the fully open position to the fully closed position inwhich the head 22| completely cuts off communication between the port221 and the valve bore 226. The reduced tapered portion 223 is made ofsuch length that the head 222 does not restrict the cylinder port 228 atany position of adjustment of the valve piston 220.

A spring 239 is positioned in the closed lower end of the valve bore 226and acts to move the valve piston 220 upwardly as far and as rapidly aspermitted by the other parts of the adjusting mechanism. The upper endof the valve piston 220 bears against the adjacent end of a plunger rod23| which projects through and is slidably supported by the cover 229which closes the valve bore 226. In order to assure quick and easymovement of the valve piston 220 and to prevent disturbance of itsposition in the valve bore 226 by lthe action of unequal fiuid pressureson the ends thereof, the portions of the valve bore 226 axially beyondthe ends of the valve piston 220 are provided with a fluid connection,such as the passage 232, and the closed lower end of the valve bore 226is connected with an exhaust passage 233 which may lead to the reservoirI2.

The outer end of the plunger rod 23| slidably bears against an adjustinglever 234 by which the position of said plunger rod 23|, and hence theposition of the valve piston 220, are determined. One end of theadjusting lever 234 is provided with a suitable handle 235, near whichsaid adjusting lever 234 is pivotally connected as at 236 with a memberwhich may be moved along acurved locking bar 231 and which may be heldor fastened at any desired position on said locking bar 231 by thefriction gripping mechanism illustrated at 238. The other end of theadjusting lever 234 is adapted to be supported upon a movable fulcrumand is accordingly provided with a slot 239 for slidably receiving afulcrum block which is rotatable upon a fulcrum pin as indicated by thedotted lines at 240.

The fulcrum pin is carried by a preferably forked fulcrum bar 24| whichis slidably supported by any suitable means, not shown, upon asupporting member 242. The fulcrum bar 24| is also attached to Atheouter end of the piston rod 243 of a compensating piston 244reciprocable in a compensating cylinder 245, the piston rod 243 beingguidingly supported by the cylinder cover 246 through which it-projects.A spring 241 is positioned in the closed end of the compensatingcylinder 245 and exerts a force tending to move the compensating piston244 toward the right as viewed in Fig. 6.

The compensating cylinder 245 is connected with anauxiliary orcompensating circuit identical with that of Fig. 3 and including aconstant capacity pump 61 receiving its supply of fluid through aconduit 63 from the same source of supply as that ofthe fluid passingthrough the metering orifice in the valve bore 226. The auxiliary orcompensating circuit also includes a variable orifice 19 inthe pumpsdischarge conduit 69, and the forward or piston rod end of thecompensating cylinder 245 is connected with said discharge conduit 69 ata point on the inlet side of said orifice 10 while the rear or closed'end of said compensating cylinder 245 is connected with said dischargeconduit 69 at a point on the outlet side of the orifice 10, theseconnections being provided by the passages 65 and 66.

With this arrangement pressure fluid from the inlet side of the orifice10 exerts a force upon the compensating piston 244 which is opposed bythe combined forces exerted upon the other side of the piston 244 by thespring 241 and fluid fromthe outlet side of the orifice 10. When thepump 61 is driven at a constant speed, the pressure drop across theorif'lce 10 will vary, as hereinbefore explained, conformably withchange in viscosity of the circulated fluid, so that the force acting tohold or move the compensating piston against the force exerted by thespring 241 will vary correspondingly. It will thus be seen that thecompensating piston 244 and the fulcrum pin will be moved toward theright upon decrease in viscosity of the circulated uid and toward theleft upon increase in said viscosity, the extent of this movementcorresponding to the extent of change in viscosity.

Such movement or change in position of the fulcrum pin causes theadjusting lever 234 to rock aboutits pivot 236, 'which in turn results`in change in position of the valve piston I224 in the valve bore 226,thus modifying the size or extent of opening of the metering orifice. In

other words, upon decrease in viscosity of the parts will move` fromtheir positions as shown vin full lines upon decrease in viscosity ofthe fluid'.

Change in resistance to flow caused by change in viscosity of the uid isthus offset and balanced by modifying the extent of opening of thevariable metering orifice to oppositely change the resistance to fiowpresented thereby.. It will therefore be seen that with properproportionofthe parts, the size or extent of opening of the metering orifice maybe modied conformably with the effect produced by change in viscosity ofthe uid so that the output of the pump I0 will not be'affected therebywhen the arrangement of Fig. 6 is substituted for. the variable orifice40 of Figs. 1, 1a and 2. It will also f be noted that the modifyingaction to compenand unchanged output of the pump for each position ofadjustment of this manually movable member.

The arrangement of Fig. 6 is suitable for use as a metering orificepositioned at any point in the discharge conduit, that is to say, thisarrangement may be employed when the metering orifice is to bepositionedintermediate the pump and the driven member, as in Fig. 1, orintermediate the driven member and the reservoir, as in Fig. 2. Theorifice 10 is preferably not adjusted during operation, but as explainedin connection with the embodiment of Fig. 3, its variable featuremakespossible the use of the same compensating mechanism with almost alliluids. A nondifferential piston may be substitutedfor the `compensatingpiston 244, although the differential areas of the pistons endsresulting from the piston rod, in the arrangement as shown, will besatisfactory in most instances because the pressure acting on the "bigend thereof is usually negligible, as it is the same as the pressureexisting on the outlet side of the orifice 10; and in fact, connectionbetween the closed end of the compensating cylinder and the outlet sideof the orifice 10 maybe omitted entirely.

Each of the springs 34 of the various embodiments functions, asdescribed above, as a constant force (i. e., a force varying only withchange in extent of compression of the spring, which is very small inthe present instance, and

determined by the rate of said spring) opposedto the resultant force dueto the difference in pressures on the opposite sides of the orifices 40,and any other suitable means for obtaining this substantially constantbut adjustable force may be employed. The spring 34 of Fig. 3 may bemade adjustable in the manner of the other ernbodiments. The doubleacting pump or equivalent multiple pump units having a common dischargeconduit, as is shown in Fig. 4, may be substituted for the single pumpsdiagrammatically illustrated in the other figures and as indicated abovethe centering mechanism shown in Fig. 4 may Ibe used in connection withany ofthe other embodiments. The constrictions 42' and 43' of Fig. 3,and shown also in other figures but not enumerated, may be employed inany of the other pipe connections shown in the drawings.

lt will be understood that the several embodiments of my invention havebeen described for the purpose of illustrating the operation and construction of the apparatus of my present invention and that changes maybe made without de-l parting from the spirit of the invention.

I claim:

l. In mechanism for controlling the output of a variable delivery pumphaving a fluid discharge conduit, a flow resistance means in saidconduit, power means for varying the delivery of said pump, and fluidpressure operated control means responsive to the pressure drop acrosssaid flow resistance means for controlling said power means, said fluidpressure operated control means being operated upon by two opposedforces, crie of said forces being the net force produced by thedifference'in pressure across said resistance flow means and tending tocontrol the power means in a direction to decrease thepump delivery andthe other of said forces being a variable force, and means for varyingthe magnitude of said variable force to thereby alter the delivery ofsaid pump.

2. In mechanism for controlling the output of a variable delivery pumphaving a fluid discharge conduit and also having a delivery-determiningmember movable to alter the delivery thereof, the combination ofpower-actuated means for moving said delivery-determining member, dualmeans for regulating at will the output of said pump, said dual meansincluding a variable oriflee in said discharge -conduit and separatemeans separately adjustable to establish a predetermined pressure dropto be maintained across said orifice, an element active by the positionthereof to control operation of said power-actuated means and correlatedwith said orifice whereby said position thereof is determined by actualpressure drop across said orifice relative to said predeterminedpressure drop, said element occupying a neutral position responsive toan actual pressure drop across said orifice equal to said predeterminedpressure drop and rendering said power-actuated means inoperative insaid neu- -tral position, said element being displaced from said neutralposition responsive to variation in said actual pressure drop relativeto said predetermined pressure drop and being active in said displacedposition to cause operation of said power-actuated meanswhereby theoutput of said pump is correct-ively modified to produce an actualpressure drop across said orifice substan- -tially equal -to saidpredetermined pressure drop, said element being restored to its neutralposition responsive to said corrective modification. of said output.

3. Control mechanism for fluid-pressure-actuated delivery-determiningmeans movable to vary the delivery of a variable delivery pump, saidpump having a fluid discharge conduit, said control mechanism comprisinga variable orifice in said discharge conduit, a valve chamber, a valveelement movable in said valve chamber and active by the position thereofto control operation of said iluid-pressure-actuated deliverydeterminingmeans by admitting pressure fluid delivered by said pump and under pumppressure to said last named means and by controlling exhaust of fluidtherefrom, iiuid connections between said discharge conduit and saidvalve chamber whereby said valve element is acted upon in one direction`by a force determined by and varying with the pressure drop across saidorifice, a spring for exerting upon said valve element a force acting ina direction opposite to that of said pressure-drop determined force,said valve element being moved responsive to difference between said twoforces acting thereon to make said fiuid-pressure-actuateddelivery-determining means operative whereby the output of said pump ismodied to produce a pressure drop across said orice of such magnitudethat the pressure-drop determined force exerted upon said valve elementequals the force exerted by said spring, said valve element being movedresponsive to said equalization of opposing forces acting thereon tomake said fluid-pressure-actuated delivery determining meansinoperative.

4. In mechanism for controlling the output of a variable delivery pumphaving a fiuid discharge conduit, a delivery-determining member for saidpump movable to vary the fluid volume delivered thereby, power-actuatedmeans for moving said delivery determining member, control meansresponsive to the rate of iiuid flow through said Vconduit forregulating the operation of said power actuated means whereby the outputof said pump is automatically varied to produce fiuid flow at asubstantially constant predeter mined rate through said conduit, andmeans for infinitely varying the output of said pump intermediate theminimum and maximum outputs thereof comprising means adjustable todetermine and to vary said predetermined rate of iluid flow and separatemeans adjustable to determine and to vary the fluid volume required toproduce said predetermined rate.

5. 1n a iiuid circuit including a variable delivery pump unit forsupplying fluid thereto and having a fiuid discharge conduit, an outputcontrol mechanism therefor comprising, a deliverydetermining membermovable to alter the delivery of said unit and power-actuated means formoving said delivery-determining member, an orifice in said dischargeconduit, said orifice being adjustable to vary the extent of openingthereof, and an element responsive to increasel and decrease indifference between the uid pressures on the inlet and outlet sidesrespectively of said orice relative to a predetermined pressure dropthere-across, said element controlling the operation of saidpower-actuated means to effect a substantially constant output of saidpump for each extent of opening said orifice and to vary the output ofsaid pump conformably with variation in adjustment of extent of openingthereof, whereby said difference in fluid pressures is maintained at anamount substantially equal to said predetermined pressure drop.

6. In fluid outputcontrol mechanism for a plurality of variable deliverypumping units having a common discharge conduit, each of said pumpingunits having a delivery-determining member independently movable to varythe delivery thereof'and also having separate poweractuated meansoperatively connected therewith for moving said delivery-determiningmeans, the combination of an orifice in said common discharge conduit,said orifice being adjustable to vary the extent of opening thereof, andan element responsive to increase and decrease in difference between thefluid pressures on the inlet and outlet sides respectively of saidorifice relative to a predetermined pressure drop thereacross, saidelement controlling the operation of all of said power-actuated means toeffect a substantially constant combined output of said pumping unitsfor each extent of opening of said orifice and also controlling theoperation of all of said power-actuated means to vary the amount of saidcombined output of said pumping units conforma-bly with variation inadjustment of extent of opening of said orince, whereby said differencein fluid pressures is maintained at an amount substantially equal tosaid predetermined pressur drop.

7. In output contr fl mechanism for a variable delivery pump laving adischarge conduit and adapted to circulate a uid of changeableviscosity, a iiow resistance means in said circuit, power means forvarying the delivery of said pump, and fluid pressure operated controlmeans responsive to the pressure drop across said flow resistance meansfor controlling said power means, said fluid pressure operated controlmeans including a valve piston operative upon a predetermined drop ofpressure across said ow resistance means to hold said power means in axed position but being operative upon departures therefrom to controlthe power means for varying the pump delivery in a direction to correctfor such departures in pressure drop, and viscosity compensating meanszo-operating with said iiuid pressure operated control means forcorrecting for the exact departures in fluid pressure drop actuallytaking place across said flow resistance means due to changes inviscosity of the fluid, including means continuously sensitive todetermine the exact extent of said departures.

8. In a fluid pressure system having a variable delivery pump and afluid circuit supplied thereby, in combination, a flow resistance meansin said circuit, means responsive to the pressure drop across said ilowresistance means for controlling the delivery of said pump, andco-operating viscosity compensating means for correcting for departuresin pressure drop across said now resistance means due to changes inviscosity irrespective of the viscosity characteristics of thecirculated fluid, including a single valve piston means directlyinfluenced both by the pressure drop across said flow resistance meansand by said co-operating viscosity compensating means.

9, In a hydrostatic iluid pressure system having a variable deliverypump in the fluid circuit thereof, in combination, a flow resistancemeans in said circuit, means responsive to the pressure drop across saidflow resistance means for controlling the delivery of said pump, andco-operating viscosity compensating means for correcting for departuresin pressure drop across said flow resistance means due to changes inviscosity, including an auxiliary circuit having means for causing fluidto flow therethrough at a constant rate and in which there is a owresistance means, together with means for utilizing the pressure dropacross said last ow resistance means in direct opposition to thepressure drop across said rst named resistance means to correct for saiddepartures in pressure drop across the first named flow resistance meansby modifying the amount of the pressure drop to be maintainedthereacross.

10. In a fluid pressure system having a variable delivery pump in thefluid circuit thereof, in combination, a flow resistance means in saidcircuit, said flow resistance means including a variable orice, meansresponsive to the pressure drop across said ow resistance means forcontrolling the delivery of said pump, and cooperating viscositycompensating means for correcting for departures in pressure drop acrosssaid flow resistance means due to changes in viscosity, including meansfor varying the extent of opening of said variable orifice in responseto changes in viscosity.

11. In uid output control mechanism for a variable delivery pump havinga fluid discharge conduit and adapted to circulate therethrough a liquidof changeable viscosity, said pump also having a delivery determiningmember movable to alter the delivery thereof and power actuated meansfor moving said delivery determining member, the combination of avariable orifice in said conduit, control mechanism regulating theoperation of said power actuated means and coordinated with saidvariable orice in pressure drop reactive relation thereto, said controlmechanism including an element movable responsive to variation in actualpressure drop across said orifice relative to a predetermined pressuredrop there-across and active upon said responsive movement to causeoperation of said power actuated means to effect corrective modificationin the delivery of said pump whereby the actual pressure drop acrosssaid orice is maintained substantially equal to said predeterminedpressure drop there-across, and means sensitive to change in viscositylof said liquid for varying the amount of said predetermined pressuredrop conformably with variation in amount of actual,

criiice in pressure drop reactive relation thereto, A

said control mechanism including an element movable responsive tovariation in actual pressure drop across said oriiice relative to apredetermined pressure drop there-across and active upon said responsivemovement to cause operation oi' said power actuated means to eiect cor-irective modification in thedelivery of said pump whereby the actualpressure drop across said orlce is maintained substantially equal tosaid predetermined pressure drop there-across, and

means lfor compensating for the efl'ect of change in viscosity o! thecirculated `fluid upon the actual pressure drop across said orificewhereby the output of said pump isunairected thereby irrespective of theviscosity characteristics of said huid.

13. In a iluid pressure system having a variable delivery pump in thefluid circuit thereof, in combination, a flow resistance means in saidcircuit, valve piston means responsive to the pressure drop across saidilow resistance means for controlling the delivery oi said pump, andcooperating viscosity compensating means for cor recting for departuresin pressure drop across said iiow resistance means due to changes inviscosity,` including an auxiliary circuit having means for causingfiuid to iiow therethrough at a constant rate and in which there is ai'low resistance means, with pipe connections leading" from both sidesoi' said last named now resistance means tosaid valve piston means.

14. In a iluid pressure system having a vari-` able delivery pump i/nthe fluid circuit thereof, in combination, -a flow 'resistance means insaid circuit, a device for varying said ow resistance means, meansresponsive to the pressure drop across said flow resistance means forcontrolling the delivery ci' said pump, and rio-operating viscositycompensating means for correcting for departures in pressure drop acrosssaid ilow resistance means due -to changes in viscosity. including meanswhereby said co-operating visv cosity compensating means reacts upon,said device which varies said ovv resistance means correcting for saiddepartures.

15. In a fluid pressure system having a varifor ablev delivery pump inthe fluid circuit thereof,

in combination, a flow resistance :means in said circuit, meansresponsive to the pressure drop across said now resistance means forcontrolling the deliveryo! said pump, and timonel-auna viscositycompensating means for correcting for departures in pressure drop acrosssaid flow ree sistance means due to changes in viscosity, including avariable cam operatively associated with the means which is responsiveto the pressure drop and active to correct for said departures.

16. In mechanism for controlling the output of a variable delivery pump,said pump having a fluid discharge conduit and also having adelivery-determining element movable to alter the delivery thereof, thecombination of power-actuated means for moving said delivery-determiningelement, an orice in said discharge A"conduit said orifice having itsoutlet side continuously under substantially atmospheric pressure, andvalve mechanism for controlling the operation of said power actuatedmeans, said valve mechanism including a valve piston movable in onedirection to cause delivery-increasing movement of said power actuatedmeans and said delivery-determining element and movable in the oppositedirec.- tion to cause delivery decreasing movement thereof, said valvepiston being acted upon by a spring urging it in its delivery increasingdirection and being oppositely acted upon by pressure fluid from theinlet side of said oriiice, urging said valve piston in itsdelivery-decreasing direction of movement.

1'7. In a fluid pressuresystem having a variable, delivery pump in thefluid circuit thereof, in combination, a flow resistance means in saidcircuit, means responsive to the pressure drop across saidflow-resistance means for controlling the delivery of said'pump, andcooperating vis-v cosity compensating means for correcting fordepartures in pressure drop across said flow resistance means due tochange in viscosity, including an auxiliary circuit having a pump forcirculating iiuid therethrough at a constant rate and in which there isan orifice the outlet side of which is continuously under substantiallyconstant pressure, with a iiuid connection leading from only the inletside of said orice to said means which is responsive to the pressuredrop across said flow resistance means.

18. Control mechanism for fluid-pressure acrtuated delivery-determiningmeans 'movable to vary the delivery of a variable delivery pump. saidpump having a fluid discharge conduit, said control mechanism includinga variable orice in said discharge conduit, a valve chamber, and a valveelement movable in said valve chamber and active by the position thereofto control opera-tion of said fluid-pressure-actuateddeliverydetermining means, means for compensating for change in pressuredrop across. said rst named orifice due to change in viscosity of thecirculated iiuid including a second orifice and also including means forcausing fluid to flow therethrough at a constant rate. uld connectionsbetween said discharge oonduit and said valve chamber whereby said valveelement is urged in one direction by difference in pressures across saidfirst named orifice and fluid connections between said vlscositycompensating means and said valve chamber whereby said-valve element isurged in the opposing direction by difference in pressures across saidsecond named oriiice.

